The present invention relates to an automatic control system for operating an electrically actuated device in response to the presence of water moisture. A specific, preferred application of the present invention is to operate a window wiper, such as a windshield wiper for a motor vehicle, in response to the presence of water moisture on the windshield. Other applications of the invention include the automatic closing of one or more power windows, a sun roof or a convertible top of a motor vehicle in response to the presence of water moisture on the vehicle.
The aforementioned U.S. Pat. No. 4,705,988 relates to an automatic window wiper control which employs, as a moisture sensor, a plurality of spaced apart, exposed conductive strips. These strips, which are preferably both linear and parallel, are positioned on a motor vehicle windshield so as to encounter, and be cleaned by a window wiper blade during its normal to and fro sweep. Connected to the moisture sensor is an electronic control circuit which converts the moisture level on successive pairs of sensor strips to a DC voltage. When the DC voltage exceeds a first threshold level, the windshield wiper motor is switched on at its normal speed. When the DC voltage exceeds a second, higher threshold level, the windshield wiper motor is caused to operate at its high speed. When the DC voltage falls below the lower threshold, the control circuit switches off the wiper motor. If the wiper motor is switched off during a wiper sweep, as is oormally the case, the normal homing circuit of the wiper motor continues to operate the motor until the wiper blade or blades reach the wiper blade home position.
While the control system disclosed in the U.S. Pat. No. 4,705,998 operates satisfactorily to detect moisture and control the wiper motor accordingly, there are instances when the moisture detection circuit does not accurately reflect the moisture condition of the windshield. For example, if one of the plurality of pairs of conductive strips of the moisture sensor are effectively "shorted" together by a single moisture droplet containing a high level of impurities, the windshield wiper will be caused to actuate even though only few moisture droplets are present on the windshield. Conversely, if exceedingly "clean" rain falls on the moisture sensor so that the resistance between the pairs of conductive strips remains high, notwithstanding the presence of substantial moisture, the control circuit will not detect the presence of such moisture and will therefore not switch on the wiper motor.
The aforementioned U.S. Pat. No. 4,705,998 discloses a moisture sensor comprising "printed circuit" type conductive strips. Since this moisture sensor is positioned where it can be wiped by the windshield wiper, it is located in the normal field of view of the driver. The aforementioned U.S. patent application Ser. No. 221,866 discloses various configurations of a moisture sensor of this type which can be located out of the normal field of view of the driver swept by the windshield wiper. In this case, however, separate means are provided for repetitively removing the moisture from the exposed surface of the moisture sensor in synchronism with the movement of the window wiper. This moisture sensor has the disadvantage of requiring a separate mechanism which not only increases the cost of the device but is subject to problems of reliability.